Early detection and correction of hidden cracks in structures such as aircraft components is important. Fatigue cracking can occur in fuselage lap joints which run longitudinally along the length of the airplane where upper skin panels overlap their adjacent lower skin panels. These panels are bonded together with adhesive and secured with multiple rows of rivets.
As a result of many pressurizations and depressurizations of the airplane, fatigue cracks can develop under and adjacent to the rivet locations. These cracks may develop below the surface of the airplane skin and thus may not be detectable by the naked eye. Furthermore, a typical commercial airliner has several thousand rivet locations. Therefore it is desirable to provide means for quickly and accurately detecting the presence of these fatigue cracks.
Historically, testing devices employing eddy currents have been used to detect structural flaws. Typically these induced eddy currents, which are present in the aluminum structure, produce magnetic fields which oppose the magnetic fields produced by the coils in the testing device. If there is a crack in the test structure, the eddy currents are disrupted causing the impedance in the tester coils to rise. This impedance change is detected by the tester and used to provide a visual indication of the crack.
A number of conventional devices for detecting hidden cracks have been disclosed. For example, in U.S. Pat. No. 4,134,067 by Woodbury, and assigned to the assignee of the present invention, discloses an eddy current device for detecting cracks in metal surfaces such as the walls of rivet holes in aircraft pans by using a rotary detection probe. Furthermore, U.S. Pat. No. 4,207,520 by Flora et al discloses a computer based device for detecting cracks under installed fasteners as a function of a phase difference between two signals.